Wear Limits Vs. Trends

John S. Evans, Wearcheck South Africa
Tags: wear debris analysis, oil analysis

Common questions asked by users of oil analysis include "What wear limits do you use?" and "What levels are normal and abnormal?"

These questions are not unreasonable. A number of original equipment manufacturers (OEMs) specify wear limits for their equipment; unfortunately, they are generally not an effective means of determining the health of a component. The levels of contamination and wear debris in an oil sample depend on too many factors for an equipment manufacturer to sort through; for example wear tables that say 0 to 50 is OK, 50 to 100 indicates a problem and over 100 is serious. This runs the risk of saying that 49 "whatevers" is acceptable and 51 is not when, in this case, there is a difference of only four percent.

Limits and Trends
The wear limit tables produced by OEMs are based on extensive research and testing by manufacturers. These people understand the equipment but, at the end of the day, the tables reflect average situations and can be used only as a guideline. They are not specifically set rules or standards. Machines rarely work in average situations, and limits determined in certain environments often have little relevance in other environments, particularly in Africa, where a minimal amount of equipment is manufactured. What holds true for North America, Western Europe or Japan might not be the case for South Africa.

The common factors that influence the concentration of wear debris and contaminants in an oil sample include:

  • type of equipment

  • environment

  • the job it is performing

  • operator skill

  • length of time the oil has been in use

  • oil consumption

Because of these factors, each piece of equipment must be treated on its own terms. It is more beneficial to assess the well-being of a machine or lubricant on the basis of a trend analysis.

Wear limits do, however, have their uses. If a diagnosis is made on the health of a component based on oil analysis and a trend is used, then a minimum of three samples is required to establish that trend. This makes one-off samples difficult to diagnose. Wear limits are useful in these situations because they provide a guideline of how the average machine should behave under normal conditions. If the unit shows signs of being critical, then the diagnosis is easy to make. However, it is the in-between cases where problems might occur, which are difficult to assess on a one-off basis without the help of a trend.

Oil analysis detects minor problems and corrects them before catastrophic failures occur. Regular oil sampling, as opposed to taking samples only when a problem arises, is important in preventing unnecessary failures. Typically a one-off sample without a trend won't supply enough information to determine the cause of a problem.

Another problem with wear limits is the range of tests they cover. Traditionally, these limits have covered only wear metals and contaminants that are detected by a spectrometer. This test provides useful information, but has one major drawback: it can detect only particles smaller than eight microns (a micron is 1/1000th of a millimeter and a human hair is about 50 microns in diameter). These particles are minute; therefore it is possible for the wear limit tables to indicate normal behavior when a severe wear situation could exist with particles greater than eight microns, which cannot be viewed by the spectrometer.

These are frequent and valid explanations taking caution when using wear limits. By using wear limits, users sometimes assume that all results from the chemical and physical tests are studied in isolation, which is an inaccurate viewpoint. The results of oil analysis must be looked at holistically. Fifty "whatevers" might be acceptable in one situation and unacceptable in another, and the only way to determine this is by studying other readings and available information. In other words, one must look at the whole picture. It is possible that identical samples from truck A and B reflect different diagnoses because the history (trend) for the two vehicles is different.

The following discussion is an example of looking holistically at trends and, in particular, results.

Dirt Entry …. Or Is It?
Dirt, grit, airborne dust - it's all the same thing and is ubiquitous. It is also damaging to machinery because if it gets into the oil, it will form a grinding paste that causes wear rates to accelerate rapidly. Fortunately for the oil analyst, dirt is composed mainly of a compound called silicon dioxide, and silicon can be easily detected in oil by spectrometric analysis.

So, would an increase in the silicon level indicate that the level of dirt entering the system is increasing? The answer is: "Yes, sometimes, but not necessarily."

Tables 1 through 4 portray a typical set of spectrometric results from an engine operating normally with no evidence of dirt entry. In Table 1, the second example shows an increase in silicon with higher wear readings. This is typical of dirt entry through the air induction system: the silicon indicates dirt during an increase in iron (liners), chrome (rings) and aluminum (pistons).

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Table 1

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Table 2

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Table 3

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Table 4

Theoretical Examples of Samples Showing High Silicon/Dirt Entry and Causes of High Copper

The third example also shows an increase in silicon, but this was caused by an internal coolant leak. When cooling water leaks into the engine, it typically evaporates. However, the additives (such as antifreeze) in the cooling water are left behind, and silicon may be part of the coolant conditioner makeup (sodium meta silicate). In this case, the sodium and copper readings increase but others do not. The sodium is also an additive and the copper is not a wear metal but has leached from the radiator core. The silicon is high, however it is a contaminant from the cooling system and not abrasive dirt.

The fourth example shows a high level of silicon, but all other readings remain more or less constant. This is an example of a silicone-based sealant or gasket compound being used. These compounds leach into the oil but do not cause any harm. If this high silicon level were caused by dirt entry, wear readings would be expected to increase due to the abrasive nature of the dirt. It should also be noted that dirt is generally a mixture of silicon oxide and aluminum oxide so, in the case of dirt entry, the aluminum level should also increase. A rough rule of thumb is the aluminum to silicon ratio for dirt varies between Al:Si = 1:10 to 1:2, depending on the component and environment. In this case, the silicon remains a contaminant but is not abrasive and is of no concern.

The fifth example shows a slight increase in the level of silicon which is caused by an additive in the oil, poly methyl siloxane, used to prevent the oil from foaming. It does not cause the wear readings to increase and, therefore, does not cause any harm.

The final set of readings in Table 1 illustrates an increase in silicon, iron, chrome and aluminum and appears similar to the second example of dirt entry through the air induction system. However, the aluminum to silicon ratio is almost 1:1, which is unusual. This is an example of piston torching. If an injector is faulty, it allows fuel to lie on top of the piston and burn. The resulting high temperatures can cause the piston to melt with the resulting increase in aluminum (piston), iron (liner) and chrome (ring). The increase in silicon is the result of silicon carbide being alloyed with the piston material in order to reduce the coefficient of expansion of the aluminum. In this case, the silicon is a wear element and is not identified due to the high level but because of the ratio of aluminum to silicon.

In Table 2, the first set of results shows a normal set of readings. The second set shows top-end dirt entry in a Detroit diesel two-stroke engine, where the iron, chrome and silicon have increased but the aluminum to silicon ratio seems incorrect and the tin level is higher. This is caused by the aluminum piston being covered with a flashing of tin to facilitate the conduction of heat.

Table 3 shows the normal readings and three examples of dirt entry through the air induction system (higher iron, chrome and aluminum) without the increase of silicon. These engines are running at Richards Bay - a mineral sands mine, a manganese mine and a chromium mine. Here, the dirt consists of the minerals that are a part of the environment where the machine is working and the silicon won't necessarily increase when dirt entry occurs.

These examples are seen on a fairly regular basis, except piston torching, as it tends to be a sudden death problem. They illustrate four situations:

  • silicon increase due to dirt entry

  • silicon increase, not due to dirt but still a problem

  • silicon increase where no problem is indicated

  • silicon remaining more or less constant when dirt entry is taking place

The only way these situations have been correctly identified is through trend analysis and viewing the results in a holistic manner. This is the reason wear limits can be misleading and at times completely wrong.

Copper Can Be Misleading Too
Silicon is a good example for illustrating the value of trending and observing the oil analysis results holistically, but an increase in readings can have different interpretations depending on other readings. Table 4 shows how an increase in copper can be caused by:

  • an abnormal wear situation

  • contamination (a problem)

  • contamination (not a problem)

  • an additive in the oil

The first example in Table 4 shows a normal set of results. The second example indicates high copper levels with an associated increase in lead, tin and iron that would typically indicate bearing wear.

The third example reveals an increase in copper due to an internal coolant leak (previously discussed with silicon). Note that the lead and tin also increase due to the use of solder in the cooling system, but because the sodium increases and the iron does not, the copper is coming from the cooling system, which indicates a problem.

The fourth example is nearly the same as the third except the sodium does not increase. In this case the copper is still coming from the cooling system but it is being leached from the oil side of the cooler. This is a common occurrence and, although it looks alarming, does not indicate a problem.

In the final example, the copper increases as does the magnesium, and the calcium decreases while there is no change in the wear readings. In this case, the copper is part of the oil additive package (an antioxidant). Wear limit tables do not take into consideration the elemental makeup of the oil used.

Generally, the spectrometric readings (elements) can be grouped into three classes: wear metals, contaminants and oil additives.

Most of the elements can belong to any one of these classes. Only by viewing the readings simultaneously can the correct class be assigned. It is important to remember that an elevated reading can be caused by more than one reason at a time.

In the example of the internal coolant leak, it is possible for other readings or tests not involving the spectrometer to be affected. An internal coolant leak may result in overheating, therefore viscosity and oxidation will increase and the base number (BN) will decrease. This is helpful because now four independent laboratory techniques will indicate the same thing.

Trends and Regular Sampling
Returning to the importance of trending and taking regular oil samples, let's reexamine the second example in Table 1 which shows an increase in silicon caused by top-end dirt entry and the associated wear of pistons, rings and liners. As dirt is by-passing the air filter, abrasive wear is occurring in the upper cylinder. Because of this, oil consumption will eventually increase. Higher oil consumption means the engine is being topped-off with fresh oil, resulting in the decrease of wear and contamination levels. Figure 1 illustrates a hypothetical situation, beginning with an increase in silicon (dirt entry) followed by an increase in iron (wear) and oil consumption, which eventually leads to a decrease in iron and silicon readings.

Following the trend of regular samples makes interpretation of the readings fairly straightforward. However, what would happen if only samples 1, 2, 3 and 10 were taken? This would show a smooth trend for iron and silicon, indicating no problem (Figure 2). Only an increase in the oil consumption would be noted, with no apparent explanation.

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Figures 1 and 2 . Wear and Dirt Entry Related to Oil Consumption

Accelerated or Abnormal?
The difference between accelerated and abnormal wear must be considered. If the lead readings were to increase in an engine sample, this typically indicates bearing wear. However, it could also be accelerated wear due to the machine working harder with all the bearings wearing a little more than usual, or it could be due to just one bearing wearing abnormally.

Many factors that affect oil analysis results are outside the control of the OEM, owner, operator and oil analyst, and these must be considered when making a diagnosis. Trending and viewing results in a holistic manner offers positive benefits for any type of condition monitoring technique, not just oil analysis. While wear limits can be beneficial, they should be used with caution. All it would require to distort a set of wear tables would be for some form of ultra filtration be fitted to filtered oil systems. The result would be the lowering of the alarm levels. One WearCheck customer has performed this across a large fleet of earthmoving equipment and the effect is noticeable. It is interesting to note that some of the strongest proponents of wear limits in the past are now acknowledging that wear readings also need to be trended.

Editor's Note:

This article was originally published by WearCheck Africa, a member of the Set Point group.


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